1. Field of the Invention
The present invention relates to a power converter, power supply and switching regulator, and more particularly, to a soft switching power converter, power supply and regulator.
2. Description of Related Art
Power converters are used to convert an unregulated power source to a regulated voltage or current source. The power converter normally includes a transformer having a primary winding and a secondary winding to provide the isolation. The switching device connected to the primary winding to control energy transferring from the primary winding to the secondary winding. The power converter operated at a high frequency allows a size and weight reduction. However, the switching behavior of the switching device generates the switching losses and electric-magnetic-interference (EMI). FIG. 1 shows a flyback power converter, and the waveforms are shown in FIG. 2. The switching device 20 is applied to switch a transformer 10 and is used to control the power delivered from the primary winding to the secondary winding of the transformer 10. The energy is stored into the transformer 10 when the switching device 20 is turned on. After the switching device 20 is switched off, the energy of the transformer 20 will be discharged to the output of the power converter through a rectifier 30. In the mean time, a reflected voltage VR is generated in the primary winding of the transformer 10 in accordance with the output voltage VO and the turn-ratio of the transformer 10. Therefore, the voltage VD across the switching device 20 is equal to the input voltage VIN plus the reflected voltage VR once the switching device 20 is turned off. The voltage VD indicates the energy stored in the parasitic capacitor 25 of the switching device 20 as well. After a discharge period TDS, the energy of the transformer 10 is fully discharged, and the energy stored in the parasitic capacitor 25 will flow back to the input voltage VIN through the primary winding of the transformer 10. The parasitic capacitor 25 (capacitance Cj) and the primary winding inductor (inductance LP) of the transformer 10 develop a resonant tank, its resonant frequency fR can be shown as equation (1),
                              f          R                =                  1                      2            ⁢            π            ⁢                                                            L                  p                                ×                                  C                  j                                                                                        (        1        )            
During the resonant, the energy of the parasitic capacitor 25 will deliver to the primary inductor of the transformer 10 back and forth. A delay time Tq is defined “from the parasitic capacitor 25 starts to discharge” “to a valley voltage occurred on the voltage VD”. The delay time Tq is the period of the quasi-resonant. It can be expressed as equation (2),
                              T          q                =                  1                      4            ×                          f              R                                                          (        2        )            
If the switching device 20 can be turned on during the valley voltage across the switching device 20, which will achieve the soft switching to minimize the switching loss and EMI.
The object of the present invention is to develop a power converter that is able to reduce the EMI and improve the efficiency of the power converter through the soft switching.